This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. NMR microscopy is a well-established technique, which employs the methods of MRI with large gradients in high magnetic fields, to achieve images with micrometer resolution. The resolution of the NMR microscope is limited to ~ 10 [unreadable]m due to the small signal from the microscopic voxels and diffusion during signal acquisition. In contrast to NMR, ESR microscopy is still at its infancy. Most of the efforts with respect to ESR imaging are directed towards low resolution imaging of large biological objects to identify the radical and the oxygen concentration. Our theoretical considerations have shown that both pulsed and CW ESR imaging methods should achieve voxel resolution better than 1[unreadable]1[unreadable]5 microns in several minutes of acquisition (at 35-60 GHz) for samples doped with stable organic trityl radicals. Such capabilities can be valuable for applications such as sub-cellular [O2] measurements, molecular imaging, which employs mobile spin probes targeted at specific molecules, functional imaging of plants, sub-cellular microviscosity measurements, exploration of radicals in materials science and other aspects addressed currently only by NMR microscopy. At present we have demonstrated an ESR imaging system, capable of acquiring 3D images with a resolution of ~10[unreadable]10[unreadable]30 microns in a few minutes of acquisition. This ESR microscope employs a commercial Continuous Wave (CW) ESR spectrometer, working at 9.1 GHz, in conjunction with a miniature imaging probe (resonator + gradient coils), gradient current drivers, and control software. The system can acquire the image of a small (~ 1.5[unreadable]1.5[unreadable]0.25 mm) sample similar to those used in optical spectroscopy either by the modulated gradient fields method, the projection reconstruction method, or by a combination of the two.